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With the exception of genetic engineering, it is nearly impossible to select a single trait or gene to be modified. While attempting to make a goldfish look more natural (or cat or dog) you will, inevitably, have an impact on behaviour.

Well known and documented example is breeding foxes for their fur. An attempt to breed them for friendliness (to make them less stressed in an enclosure with other foxes, and to make them easier to care for) had the unintended, and undesired effect of changing their tails, ears and fur colour.

You may want your goldfish to live longer, or look more like a carp, but keeping a carp in aquaria is near impossible. They are not bred to be in enclosed spaces. Breeding them to not be overly stressed and breed in a fish tank has the side-effect of making them change colour, become rounder, etc.

Likewise, breeding a cat to have an ideal body shape and physical fitness would result in a feral or outdoor cat, unhappy and unwilling to to locked in a house for its entire life. This is not to defend the extremes that fancy breeders go to, however the polar opposite is equally untenable.

Another thing to consider is that many extreme fancy goldfish are not the result of breeding, but of grooming. Groomers feed their fish high concentrations of high protein feed. This is like feeding a child 20 cheeseburgers a day, and blaming his odd round shape on his genes.

There is truth in what you say. But they're extreme examples. All I am saying is that the bulbous bodies, laboured swimming and double tails of fancy goldfish are not for me.

I'm very familiar with the Belyaev fox breeding experiment. Anyone who is not familiar with it should check it out. Fascinating, evolution in action...and the experiment continues to this day.